Advances and challenges of hydrogel electrolytes for rechargeable zinc-iodine batteries

Zinc-iodine batteries (ZIBs) show great promise for safe, economical energy storage, benefiting from their high theoretical capacity and inherent safety. However, their practical deployment is severely limited by rapid capacity decay and premature cell failure, primarily originating from the polyiodide shuttling effect at the iodine cathode and interfacial instabilities at the zinc (Zn) anode, including dendrite growth and parasitic side reactions. Recently, functionalized hydrogel electrolytes (HEs) have attracted increasing attention as an effective strategy to address these challenges. This review comprehensively summarizes the design principles and working mechanisms of HEs in ZIBs, focusing specifically on their ability to suppress polyiodide shuttling through electrostatic regulation, chemical anchoring, and synergistic confinement effects, while simultaneously stabilizing Zn anodes by modulating ion transport, interfacial chemistry, and mechanical constraints. The underlying physicochemical mechanisms and representative implementation strategies are critically discussed. Finally, the current limitations of HEs in ZIBs are analyzed, and future research directions are suggested, including the enhancement of multi-electron iodine chemistries, wide-temperature operation, and multifunctional hydrogel systems. This review focuses on providing mechanistic insights and rational design guidelines for advancing high-performance ZIBs.